Practice golf ball

ABSTRACT

A practice golf ball has a core made of a rubber composition which includes a base rubber, a co-crosslinking agent, a crosslinking initiator and a metal oxide, and has a cover which encases the core and is made of a resin material. The co-crosslinking agent is methacrylic acid. The resin material has a breaking strength of 20 to 80 MPa and an elongation of 150 to 600%. The ball is endowed with the properties required of practice balls intended for long-term use, including better durability to cracking and durability of appearance than ordinary game balls, and also better durability to ball surface loss than the one-piece golf balls which are commonly used as practice balls.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2011-099853 filed in Japan on Apr. 27, 2011,the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a practice golf ball which is suitablefor use at such places as driving ranges. More specifically, theinvention relates to a practice golf ball endowed with the propertiesrequired of practice balls intended for long-term use, including betterdurability to cracking and durability of appearance than ordinary gameballs, and also better durability to ball surface loss than theone-piece golf balls which are commonly used as practice balls.

2. Prior Art

In terms of the performance sought in a practice golf ball, it isgenerally desirable to obtain ball characteristics which are nodifferent from those of golf balls used to play an actual round of golf.If, for example, a practice ball has a feel on impact or a flightperformance which differs from that of a game ball, when the time comesto play an actual round, the golfer will be unable to take fulladvantage of the skills acquired through practice.

On the other hand, because of the size limitations of driving ranges,there is a need today for low-distance practice golf balls in order tokeep the balls from flying out of the driving range. Accordingly, thereexists a desire for practice balls which have a performancecharacterized by the same feel on impact as a game ball, but a shortflight distance.

Moreover, because practice golf balls are used over and over again at adriving range, in order to reduce the costs of operating the range, itis desirable for the balls to be capable of enduring use for as long aperiod of time as possible even when repeatedly hit. That is, becausethe golf balls at a driving range are repeatedly used over a long periodof time by many golfers practicing their skills, there is a desire forsuch practice balls to have a durability, including a durability tocracking, which is superior to that of ordinarily used game balls.

As for the ball structure in a practice golf ball, for the ball to beimparted with a flight performance and feel similar to those experiencedwhen playing an actual round of golf, it is more desirable to use atwo-piece solid golf ball than a one-piece ball.

As is widely known, two-piece solid golf balls are composed of a coreand a cover, with the core being a rubber crosslinked material ofcertain desirable properties obtained by using a base rubber composedprimarily of cis-1,4-polybutadiene rubber to which compoundingingredients such as a co-crosslinking agent, a metal oxide and anorganic peroxide have been added. For example, JP-A 59-49779 teaches, asthe rubber composition for the core of a two-piece solid golf ball, thecompounding of a given amount of zinc methacrylate as a co-crosslinkingagent in cis-1,4-polybutadiene rubber. However, when zinc methacrylateis used in this way in a core-forming rubber composition, achieving goodball durability in long-term use at a golf driving range has beendifficult.

In addition, JP-A 2003-70936, JP-A 2007-61614, JP-A 2007-301357, JP-A2010-115485, JP-A 2010-115486, JP-A 2004-180793, JP-A 2008-149190, JP-A2009-195761, JP-A 2005-27814, and JP-A 2010-269147 all describe, asrubber compositions for the cores of two-piece solid golf balls, thecompounding of given amounts of zinc acrylate in cis-1,4-polybutadienerubber. However, here too, when zinc acrylate is used in therubber-forming rubber composition, achieving good ball durability inlong-term use at a driving range has been difficult.

An example of the prior art relating to this invention is described inJP-A 2002-126128, which is directed at a one-piece golf ball having anoptimized internal hardness profile from the surface toward the centerof the ball. However, this prior-art golf ball lacks a satisfactorydurability to surface loss when hit with the sharp portion of aclubhead, such as the leading edge of an iron. Here and below, “surfaceloss” refers to the loss of material from the surface of a golf ballwhen struck with a golf club.

In view of the foregoing, it is an object of the present invention toprovide a practice golf ball endowed with the properties required ofpractice balls intended for long-term use, including better durabilityto cracking and durability of appearance than ordinary game balls, andalso better durability to ball surface loss than the one-piece golfballs which are commonly used as practice balls.

SUMMARY OF THE INVENTION

We have discovered that, in the production of practice golf balls havinga core and a cover, by using methacrylic acid as a co-crosslinking agentfor the base rubber in the core-forming rubber composition and by usinga resin material having a breaking strength of from 20 MPa to 80 MPa andan elongation of from 150% to 600% as the cover material, owing to thesynergistic effects of these materials, the resulting balls have betterdurability to cracking and durability to abrasion than anticipated bygolf ball designers. This discovery has made it possible to obtainpractice golf balls which, even with long-term use, retain a goodappearance and a good flight performance, and moreover have betterdurability to ball surface loss than the one-piece golf balls which arecommonly used as practice balls.

That is, in the present invention, by using methacrylic acid as aco-crosslinking agent in the core-forming rubber composition and byoptimizing the amounts of methacrylic acid and crosslinking initiatorincluded in the composition, the ball can be made much more durable tocracking than a game ball. In addition, by selecting a resin materialwhich has a high strength and good elongation and using this resinmaterial to form the cover, a practice golf ball endowed with excellentdurability to cracking, durability to surface loss and durability toabrasion can be obtained. Moreover, in the invention, by optimizing thecross-sectional shape of the dimples formed on the ball surface, apractice golf ball having excellent durability to markings (e.g., abrand name or player number printed on the surface of the ball at thetime of manufacture) in long-term use can be obtained. Furthermore, byoptimizing the internal hardness profile of the core, a golf ball havinga good feel on impact can be obtained.

Accordingly, the invention provides a practice golf ball having a coremade of a rubber composition which includes a base rubber and, ascompounding ingredients: a co-crosslinking agent, a crosslinkinginitiator and a metal oxide; and having a cover which encases the coreand is made of a resin material. The co-crosslinking agent ismethacrylic acid. The resin material has a breaking strength of from 20MPa to 80 MPa and an elongation of from 150% to 600%.

The practice golf ball of the present invention preferably has aninitial velocity of not more than 76 m/s.

In the practice golf ball of the invention, the resin material making upthe cover may be composed primarily of polyurethane, the cover materialmay have a Shore D hardness of from 30 to 57, and the cover may have athickness of from 0.3 mm to 2.5 mm.

The practice golf ball of the invention may have formed on a surfacethereof a plurality of dimples, each dimple having a spatial volumebelow a flat plane circumscribed by an edge of the dimple, and the sumof the dimple spatial volumes, expressed as a percentage (VR) of thevolume of a hypothetical sphere representing the ball were the ball tohave no dimples on the surface thereof, being from 0.8% to 1.7%.

The practice golf ball of the invention may have formed on a surfacethereof a plurality of dimples which satisfy conditions (1) and (2)below:

(1) the dimples have a peripheral edge provided with a roundnessrepresented by a radius of curvature R of from 0.5 mm to 2.5 mm; and

(2) the ratio ER of a collective number of dimples RA having a radius ofcurvature R to diameter D ratio (R/D) of at least 20%, divided by atotal number of dimples N on the surface of the ball, is from 15% to95%.

The practice golf ball which satisfies above conditions (1) and (2)preferably satisfies also condition (3) below:

(3) the ball has thereon a plurality of dimple types of differingdiameter, and the ratio DER of a combined number of dimples DE obtainedby adding together dimples having an own diameter and an own radius ofcurvature larger than or equal to a radius of curvature of dimples oflarger diameter than the own diameter plus dimples of a type having alargest diameter, divided by the total number of dimples N on thesurface of the ball, is at least 80%.

The practice golf ball which satisfies above conditions (1) to (3)preferably satisfies also conditions (4) to (6) below:

(4) the number of dimples types of differing diameter is 3 or more;

(5) the total number of dimples N is not more than 380; and

(6) the surface coverage SR of the dimples, which is the sum ofindividual dimple surface areas, each defined by a flat planecircumscribed by an edge of the dimple, expressed as a percentage of thesurface area of a hypothetical sphere representing the ball were theball to have no dimples on the surface thereof, is from 60% to 74%.

Preferably, in the practice golf ball of the invention, the core has ahardness profile in which, letting A be the JIS-C hardness at a surfaceof the core, B be the JIS-C hardness at a position 2 mm inside the coresurface, C be the JIS-C hardness at a position 5 mm inside the coresurface, D be the JIS-C hardness at a position 10 mm inside the coresurface, E be the JIS-C hardness at a position 15 mm inside the coresurface, and F be the JIS-C hardness at a center of the core: A is from60 to 88, B is from 54 to 83, C is from 56 to 85, D is from 54 to 80, Eis from 51 to 75, and F is from 48 to 72; the relative hardnessconditions A>B<C≧D>E>F are satisfied; the value A−F is not more than 19;the core is formed in such a way that A has the highest value among A toF; the value A−C is from 0 to 8; the core has a specific gravity of from1.05 to 1.2; and, when the core and the ball are each compressed under afinal load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf),letting deflection by the core be CH and deflection by the ball be BH,the core deflection CH is from 2.0 mm to 7.0 mm, the ball deflection BHis from 2.0 mm to 7.0 mm, and the ratio CH/BH is from 0.95 to 1.1

In the practice golf ball of the invention, the metal oxide ispreferably zinc oxide, and the compounding ingredients are preferablyincluded in respective amounts of from 10 to 40 parts by weight ofmethacrylic acid, from 0.3 to 5.0 parts by weight of crosslinkinginitiator, and from 15 to 30 parts by weight of zinc oxide, per 100parts by weight of the base rubber.

The practice golf ball of the invention is endowed with the propertiesrequired of practice balls intended for long-term use, including betterdurability to cracking and durability of appearance than ordinary gameballs, and also better durability to ball surface loss than theone-piece golf balls which are commonly used as practice balls.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic cross-sectional diagram of a practice golf ballaccording to one embodiment of the invention.

FIG. 2 is a schematic diagram of a core illustrating positions A to F inthe core hardness profile.

FIG. 3 is a schematic diagram showing an example of a dimplecross-section.

FIG. 4A is a top view and FIG. 4B is a side view showing an example of adimple configuration.

FIG. 5 is a top view showing the markings that were placed on the golfballs fabricated in the examples and the comparative examples.

DETAILED DESCRIPTION OF THE INVENTION

The objects, features and advantages of the invention will become moreapparent from the following detailed description, taken in conjunctionwith the foregoing diagrams.

The structure of the practice golf ball of the invention is exemplifiedby, as shown in FIG. 1, a two-piece solid golf ball G having a core 1and a cover 2 which encases the core 1. The cover 2 typically has aplurality of dimples D formed on a surface thereof. In the diagram, thecore 1 and the cover 2 are shown as single layers, although the core andthe cover may each be composed of a plurality of layers. Also, althoughit is not shown in Figs, a paint layer such as a clear paint may beformed to the surface of the ball.

The core is obtained by vulcanizing a rubber composition composedprimarily of a rubber material. The rubber composition used to form thecore includes a base rubber, a co-crosslinking agent, a crosslinkinginitiator, a metal oxide and, optionally, an antioxidant. The baserubber used in this rubber composition is preferably polybutadiene. Inthe invention, as will be subsequently described, it is preferable forthe core cross-sectional hardness to change in specific ways from thesurface to the center of the core, and for the core cross-sectionalhardness distribution, also referred to herein as the “core hardnessprofile,” to be adjusted within certain desired ranges. To this end, informulating the core, it is essential to suitably adjust, for example,the amounts in which the various subsequently described compoundingingredients are included, the vulcanization temperature and thevulcanization time.

The polybutadiene used as the rubber component must have a cis-1,4 bondcontent of at least 60 wt %, preferably at least 80 wt %, morepreferably at least 90 wt %, and most preferably at least 95 wt %. Ifthe cis-1,4 bond content is too low, the rebound may decrease. Inaddition, the polybutadiene has a 1,2-vinyl bond content of preferably 2wt % or less, more preferably 1.7 wt % or less, and even more preferably1.5 wt % or less.

The polybutadiene has a Mooney viscosity (ML₁₊₄, (100° C.)) which ispreferably at least 30, more preferably at least 35, even morepreferably at least 40, and most preferably at least 45, but ispreferably not more than 100, more preferably not more than 80, evenmore preferably not more than 70, and most preferably not more than 60.

The term “Mooney viscosity” used herein refers to an industrialindicator of viscosity (JIS K6300) as measured with a Mooney viscometer,which is a type of rotary plastometer. This value is represented by theunit symbol ML₁₊₄ (100° C.), wherein “M” stands for Mooney viscosity,“L” stands for large rotor (L-type), and “1+4” stands for a pre-heatingtime of 1 minute and a rotor rotation time of 4 minutes. The “100° C.”indicates that measurement was carried out at a temperature of 100° C.

In order to obtain the rubber composition in a molded and vulcanizedform which has a good rebound, it is preferable for the polybutadiene tohave been synthesized using a rare-earth catalyst or a Group VIII metalcompound catalyst.

The rare-earth catalyst is not subject to any particular limitation,although preferred use can be made of a catalyst which employs alanthanum series rare-earth compound. Also, where necessary, anorganoaluminum compound, an alumoxane, a halogen-bearing compound and aLewis base may be used in combination with the lanthanum seriesrare-earth compound. Preferred use can be made of, as the various abovecompounds, those compounds mentioned in JP-A 11-35633, JP-A 11-164912and JP-A 2002-293996.

Of the above rare-earth catalysts, the use of a neodymium catalyst thatemploys a neodymium compound, which is a lanthanide series rare-earthcompound, is especially recommended. In such a case, a polybutadienerubber having a high cis-1,4 bond content and a low 1,2-vinyl bondcontent can be obtained at an excellent polymerization activity.

The polybutadiene has a polydispersity Mw/Mn (Mw being theweight-average molecular weight, and Mn being the number-averagemolecular weight) of preferably at least 2.0, more preferably at least2.2, even more preferably at least 2.4, and most preferably at least2.6. The upper limit is preferably 6.0 or less, more preferably 5.0 orless, and even more preferably 4.5 or less. If Mw/Mn is too low, theworkability may decrease. On the other hand, if Mw/Mn is too high, therebound may decrease.

When the above polybutadiene is used as the base rubber, the proportionof the overall rubber represented by the polybutadiene, although notsubject to any particular limitation, is preferably at least 40 wt %,more preferably at least 60 wt %, even more preferably at least 80 wt %,and most preferably at least 90 wt %. The above polybutadiene mayrepresent fully 100 wt % of the base rubber, although 98 wt % or less ispreferred, and 95 wt % or less is more preferred.

Illustrative examples of cis-1,4-polybutadiene rubbers which may be usedinclude the high-cis products BR01, BR11, BR02, BR02L, BR02LL, BR730 andBR51, all of which are available from JSR Corporation.

Rubber components other than the above polybutadiene may also be used inthe base rubber, insofar as the objects of the invention are attainable.Illustrative examples of rubber components other than the abovepolybutadiene include polybutadienes other than the above polybutadiene,and other diene rubbers such as styrene-butadiene rubbers, naturalrubbers, isoprene rubbers and ethylene-propylene-diene rubbers.

Isoprene rubbers which may be used include those having a cis-1,4 bondcontent of at least 60 wt %, preferably at least 80 wt %, and morepreferably at least 90 wt %, and having a Mooney viscosity (ML₁₊₄ (100°C.)) of at least 70, preferably at least 75, and more preferably atleast 80, with an upper limit of 90 or less, and preferably 85 or less.For example, the product IR2200 available from JSR Corporation may beused.

Styrene-butadiene rubbers which may be used include solution-polymerizedstyrene-butadiene rubbers and emulsion-polymerized styrene-butadienerubbers. Compared with emulsion-polymerized styrene-butadiene rubbers,solution-polymerized styrene-butadiene rubbers do not contain organicacids and low-molecular-weight components that arise from themanufacturing process, and thus have a poor processability. In addition,when a solution-polymerized butadiene-styrene rubber is used in acore-forming rubber composition, the seasonal (winter versus summer)difference in ball rebound is larger. On the other hand, when anemulsion-polymerized styrene-butadiene rubber is used in thecore-forming rubber composition, compared with when asolution-polymerized styrene-butadiene rubber is used, hardening of thecore due to temperature changes on account of seasonal differences canbe effectively prevented. Therefore, by using these two types ofstyrene-butadiene rubber having different qualities in a specific ratio,it is sometimes possible to suppress a decline in resilience and achange in hardness during winter use while maintaining a goodprocessability. By way of illustration, use may be made of thesolution-polymerized products SBR-SL552, SBR-SL555 and SBR-SL563(available from JSR Corporation) as the solution-polymerizedstyrene-butadiene rubber, and use can be made of theemulsion-polymerized products SBR 1500, SBR 1502, SBR 1507 and SBR 0202(available from JSR Corporation) as the emulsion-polymerizedstyrene-butadiene rubber. Ordinary, commercially available,solution-polymerized styrene-butadiene rubber has a styrene bond contentof from 5 wt % to 50 wt %, and emulsion-polymerized styrene-butadienerubber has a styrene bond content of from 15 wt % to 50 wt %.

The proportion of the overall rubber represented by rubber componentsother than polybutadiene is preferably 0 wt % or more, more preferablyat least 2 wt %, and most preferably at least 5 wt %, but is preferablynot more than 60 wt %, more preferably not more than 40 wt %, even morepreferably not more than 20 wt %, and most preferably not more than 10wt %.

In the invention, methacrylic acid is an essential ingredient which isemployed as the co-crosslinking agent. Methacrylic acid is included inan amount, per 100 parts by weight of the base rubber, of preferably atleast 10 parts by weight, more preferably at least 11 parts by weight,even more preferably at least 12 parts by weight, and most preferably atleast 13 parts by weight. The upper limit in the amount of methacrylicacid is preferably not more than 40 parts by weight, more preferably notmore than 35 parts by weight, even more preferably not more than 30parts by weight, and most preferably not more than 25 parts by weight.Including too much methacrylic acid may make the core too hard, givingthe ball an unpleasant feel on impact. On the other hand, including toolittle methacrylic acid may make the core too soft, also giving the ballan unpleasant feel on impact.

It is preferable to use an organic peroxide as the crosslinkinginitiator in this invention. Examples of commercial products that may beadvantageously used include Percumyl D, Perhexa 3M and Perhexa C40, allavailable from NOF Corporation. These may be used singly or ascombinations of two or more thereof.

The crosslinking initiator is included in an amount, per 100 parts byweight of the base rubber, of preferably at least 0.3 part by weight,more preferably at least 0.6 part by weight, and even more preferably atleast 0.9 part by weight. The upper limit in the amount of crosslinkinginitiator is preferably not more than 5.0 parts by weight, morepreferably not more than 4.0 parts by weight, even more preferably notmore than 3.0 parts by weight, and most preferably not more than 2.0parts by weight. Including too much crosslinking initiator may make thecore too hard, giving the ball an unpleasant feel on impact and alsosubstantially lowering the durability to cracking. On the other hand,including too little crosslinking initiator may make the core too soft,giving the ball an unpleasant feel on impact and also substantiallylowering productivity.

It is preferable to use zinc oxide as the metal oxide in this invention,although metal oxides other than zinc oxide may be used insofar as theobjects of the invention are attainable. The metal oxide is included inan amount, per 100 parts by weight of the base rubber, of preferably atleast 15 parts by weight, more preferably at least 17 parts by weight,even more preferably at least 19 parts by weight, and most preferably atleast 21 parts by weight. The upper limit in the amount of metal oxideis preferably not more than 30 parts by weight, more preferably not morethan 28 parts by weight, even more preferably not more than 26 parts byweight, and most preferably not more than 24 parts by weight. Includingtoo much or too little may make it impossible to obtain a suitableweight and a suitable hardness and rebound.

In the practice of the invention, an antioxidant may also be included inthe rubber composition. For example, use may be made of the commercialproducts Nocrac NS-6, Nocrac NS-30 and Nocrac 200, all available fromOuchi Shinko Chemical Industry Co., Ltd. These may be used singly or ascombinations of two or more thereof.

The amount of antioxidant included per 100 parts by weight of the baserubber, although not subject to any particular limitation, is preferablyat least 0.1 part by weight, and more preferably at least 0.2 part byweight, but is preferably not more than 1.0 part by weight, morepreferably not more than 0.7 part by weight, and even more preferablynot more than 0.4 part by weight. Including too much or too littleantioxidant may make it impossible to achieve a suitable core hardnessgradient, as a result of which a good rebound, good durability and goodspin rate-lowering effect on full shots may not be achieved.

In the practice of the invention, from a resource recycling standpoint,a ground or abraded powder of vulcanized rubber may be included in asmall amount of 40 is parts by weight or less per 100 parts by weight ofthe base rubber. In such a case, the ground or abraded powder may becompounded in an amount, per 100 parts by weight of the base rubber,which is more than 0 wt %, preferably at least 2 wt %, and mostpreferably at least 5 wt %, but is preferably not more than 40 wt morepreferably not more than 35 wt %, even more preferably not more than 30wt %, and most preferably not more than 25 wt %. The ground or abradedpowder of vulcanized rubber is a vulcanizate which contains rubber andunsaturated carboxylic acid or a metal salt thereof. It is desirable forthe ground or abraded powder of vulcanized rubber used to have aparticle size which is preferably at least 20 μm, more preferably atleast 25 μm, and most preferably at least 30 μm, but is preferably notmore than 1,000 μm, more preferably not more than 900 μm, and mostpreferably not more than 800 μm. Adding a crushed or abraded powder ofvulcanized rubber has such effects as improving the productivity of thevulcanizate and increasing the durability to cracking. However,including too much may markedly lower the workability of the rubbercomposition and the productivity.

The core may be produced by using a known method to vulcanize and curethe rubber composition containing the various above ingredients. Forexample, production may be carried out by using a mixing apparatus suchas a Banbury mixer or a roll mill to mix the rubber composition,compression molding or injection molding the mixed composition in a coremold, then curing the molded body by suitable heating at a temperaturesufficient for the organic peroxide and co-crosslinking agent to act,such as under conditions of between about 100° C. and about 200° C. fora period of from about 10 minutes to about 40 minutes. The core hardnessprofile of the invention may be achieved by a combination of thevulcanization conditions and adjustment of the rubber formulation.

The core diameter, although not subject to any particular limitation, ispreferably at least 38.0 mm, more preferably at least 38.9 mm, and evenmore preferably at least 39.3 mm, but is preferably not more than 42.1mm, and more preferably not more than 41.1 mm. At a core diameteroutside of this range, the durability of the ball to cracking may worsendramatically and the initial velocity of the ball may decrease.

It is recommended that the specific gravity of the core be at least1.05, preferably at least 1.08, and more preferably at least 1.1, butnot more than 1.2, preferably not more than 1.15, and more preferablynot more than 1.13.

The core deflection under loading (referred to here and below as “CH”),i.e., the deflection by the core when compressed under a final load of1,275 N (130 kgf) from an initial load of 98 N (10 kgf), is preferablyat least 2.0 mm, more preferably at least 2.3 mm, and even morepreferably at least 2.4 mm, but is preferably not more than 7.0 mm, morepreferably not more than 6.0 mm, even more preferably not more than 5.0mm, and most preferably not more than 4.5 mm. If the core deflection CHis too small, the feel of the practice golf ball on impact may be sohard as to make the ball unpleasant to use. On the other hand, if thecore deflection is too large, the feel of the practice golf ball onimpact may be so soft as to make the ball unpleasant to use, in additionto which the productivity may decline considerably.

In the practice of the invention, as shown in the schematic diagram ofthe core in FIG. 2, letting A be the JIS-C hardness at a surface of thecore, B be the JIS-C hardness at a position 2 mm inside the coresurface, C be the JIS-C hardness at a position 5 mm inside the coresurface, D be the JIS-C hardness at a position 10 mm inside the coresurface, E be the JIS-C hardness at a position 15 mm inside the coresurface, and F be the JIS-C hardness at the center of the core, therespective values A to F, although not subject to any particularlimitations, preferably fall within the specific ranges indicated below.By thus setting the hardness profile at the core interior withinspecific ranges, both a comfortable feel on impact similar to that of agame ball and also a good durability to cracking can be obtained.

Letting A be the JIS-C hardness at the surface of the core, the value ofA is preferably at least 60, more preferably at least 63, and even morepreferably at least 65, but is preferably not more than 88, morepreferably not more than 86, and even more preferably not more than 84.

Letting B be the JIS-C hardness at a position 2 mm inside the coresurface, the value of B is preferably at least 54, more preferably atleast 57, and even more preferably at least 59, but is preferably notmore than 83, more preferably not more than 81, and even more preferablynot more than 79.

Letting C be the JIS-C hardness at a position 5 mm inside the coresurface, the value of C is preferably at least 56, more preferably atleast 59, and even more preferably at least 61, but is preferably notmore than 85, more preferably not more than 83, and even more preferablynot more than 81.

Letting D be the JIS-C hardness at a position 10 mm inside the coresurface, the value of D is preferably at least 54, more preferably atleast 57, and even more preferably at least 60, but is preferably notmore than 80, more preferably not more than 78, and even more preferablynot more than 76.

Letting E be the JIS-C hardness at a position 15 mm inside the coresurface, the value of E is preferably at least 51, more preferably atleast 54, and even more preferably at least 57, but is preferably notmore than 75, more preferably not more than 73, even more preferably notmore than 71, and most preferably not more than 70.

Letting F be the JIS-C hardness at the center of the core, the value ofF is preferably at least 48, more preferably at least 51, and even morepreferably at least 54, but is preferably not more than 72, morepreferably not more than 70, and even more preferably not more than 68.

Moreover, in the above core hardness profile, it is preferable for therelative hardness conditions A>B<C≧D>E>F to be satisfied, for the valueA−F to be not more than 19, for the core to be formed in such a way thatA has the highest value among A to F, and for the value A−C to be in arange of from 0 to 8. If the above conditions are not satisfied, theball may have a diminished feel on impact and a reduced durability tocracking.

The value of A−C is preferably within a range of from 0 to 8, with thelower limit being preferably at least 0, and more preferably at least 1,and the upper limit being preferably not more than 8, more preferablynot more than 6, and even more preferably not more than 4. The value ofA−F is preferably at least 3, more preferably at least 6, and even morepreferably at least 8, but is preferably not more than 19.

In the practice of the invention, the core may be administered surfacetreatment with a solution containing a haloisocyanuric acid and/or ametal salt thereof.

Prior to surface-treating the core with a solution containing ahaloisocyanuric acid and/or a metal salt thereof, adhesion between thecore surface and the adjoining cover material can be further enhanced byabrading the surface of the core (referred to below as “surfacegrinding”).

Such surface grinding removes the skin layer from the surface of thevulcanized core, and thus makes it possible to enhance the ability ofthe solution of haloisocyanuric acid and/or a metal salt thereof topenetrate the core surface and also to increase the surface area ofcontact with the adjoining cover material. Exemplary surface grindingmethods include buffing, barrel grinding and centerless grinding.

The above haloisocyanuric acid and metal salt thereof is the compoundshown in the following formula (I).

In the formula, X is a hydrogen atom, a halogen atom or an alkali metalatom. At least one occurrence of X is a halogen atom. Preferred halogenatoms include fluorine, chlorine and bromine, with chlorine beingespecially preferred. Preferred alkali metal atoms include lithium,sodium and potassium.

Illustrative examples of the haloisocyanuric acid and/or a metal saltthereof include chloroisocyanuric acid, sodium chloroisocyanurate,potassium chloroisocyanurate, dichloroisocyanuric acid, sodiumdichloroisocyanurate, sodium dichloroisocyanurate dihydrate, potassiumdichloroisocyanurate, trichloroisocyanuric acid, tribromoisocyanuricacid, dibromoisocyanuric acid, bromoisocyanuric acid, sodium and othersalts of dibromisocyanuric acid, as well as hydrates thereof, anddifluoroisocyanuric acid. Of these, chloroisocyanuric acid, sodiumchloroisocyanurate, potassium chloroisocyanurate, dichloroisocyanuricacid, sodium dichloroisocyanurate, potassium dichloroisocyanurate andtrichloroisocyanuric acid are preferred because they are readilyhydrolyzed by water to form acid and chlorine, and thus play the role ofinitiating addition reactions to the double bonds in the diene rubbermolecules. The use of trichloroisocyanuric acid provides an especiallyoutstanding adhesion-improving effect.

The haloisocyanuric acid and/or a metal salt thereof is preferablydissolved in an organic solvent and used as a solution. A known organicsolvent may be used for this purpose, with the use of an organic solventwhich is soluble in water being especially preferred. Examples includeethyl acetate, acetone and methyl ethyl ketone. Of these, acetone isespecially preferred on account of its ability to penetrate the coresurface. The use of a water-soluble solvent is preferable because suchsolvents readily take up moisture; either the moisture which has beentaken up readily undergoes a hydrolysis reaction with thehaloisocyanuric acid and/or a salt thereof deposited on the core surfaceor, when water washing is used in a subsequent step, as the affinity tothe core surface increases, a hydrolysis reaction between the water andthe haloisocyanuric acid and/or a metal salt thereof more readilyarises.

When dissolved in an organic solvent, the content of the haloisocyanuricacid and/or a metal salt thereof in the solution is preferably at least0.3 wt %, more preferably at least 1 wt %, and more preferably at least2.5 wt %. At less than 0.3 wt %, the adhesion improving effectanticipated following core surface treatment may not be obtained,possibly resulting in a poor durability to impact. The upper limit inthe content may be as high as the saturated solution concentration.However, from the standpoint of cost effectiveness, when prepared as anacetone solution, for example, setting the upper limit in content to 10wt % is preferred. The core is immersed in the solution for a length oftime which is preferably at least 0.3 second, more preferably at least 3seconds, and even more preferably at least 10 seconds, but is preferablynot more than 5 minutes, more preferably not more than 1 minute, andeven more preferably not more than 30 seconds. If the immersion time istoo short, the desired effects of treatment may not be obtained, whereasif the immersion time is too long, a loss in ball productivity mayoccur.

The method of treating the core surface with a haloisocyanuric acidand/or a metal salt thereof is exemplified by methods which involvecoating the surface of the core with a solution of haloisocyanuric acidand/or a metal salt thereof by brushing or spraying on the solution, andmethods in which the core is immersed in a solution of thehaloisocyanuric acid and/or a metal salt thereof. From the standpoint ofproductivity and high penetrability of the core surface by the solution,the use of an immersion method is especially preferred.

After the core has been surface treated with a solution containinghaloisocyanuric acid and/or a metal salt thereof, it is preferable towash the surface of the core with water. Water washing of the coresurface may be carried out by a method such as running water, spraying,or soaking in a washing tank. However, because the aim here is notmerely to wash, but also to initiate and promote the desired treatmentreactions, too vigorous a washing method will not be appropriate.Therefore, preferred use may be made of washing by soaking in a washingtank. In such a case, it is desirable to place the cores to be washedfrom about one to five times in a washing tank that has been filled withfresh water.

Treating the core surface with a haloisocyanuric acid and/or a metalsalt thereof greatly improves adhesion between the core surface and thecover. The reason for this is not well understood, but is thought to beas follows.

The haloisocyanuric acid and/or a metal salt thereof, together with thesolvent, penetrates to the interior of the diene rubber making up thecore and approaches the vicinity of the double bonds on the backbone.Water then enters the core surface, whereupon the haloisocyanuric acidand/or a metal salt thereof is hydrolyzed by the water, releasing thehalogen. The halogen attacks the double bonds on the diene rubberbackbone located nearby, as a result of which an addition reactionproceeds. In the course of this addition reaction, the liberatedisocyanuric acid is added, together with the halogen, to the dienerubber backbone while retaining the cyclic structure. The addedisocyanuric acid has three —NHCO— structures on the molecule.

Because —NHCO— structures are thereby conferred to the core surface thathas been treated with the haloisocyanuric acid and/or a metal saltthereof, adhesion with the cover material improves further. It is mostlikely because of this that the durability of the golf ball to impactimproves. Moreover, when a polyurethane elastomer or polyamide elastomerhaving the same —NHCO— structures on the polymer molecule is used as thecover material, the affinity increases even further, presumablyincreasing the durability to impact.

When the addition of isocyanuric acid and chlorine (as the halogen) tothe surface of diene rubber has occurred, changes in the bonding statesbefore and after addition appear in an infrared absorption spectrum asincreases in the C═O bond (stretching) absorption peak at 1725 to 1705cm⁻¹, the broad H—H bond (stretching) absorption peak at 3450 to 3300cm⁻¹, and the C—Cl bond absorption peak at 800 to 600 cm⁻¹. Hence, bymeasuring the IR absorption spectrum of a surface-treated core andconfirming increases in these absorption peaks, it is possible toqualitatively confirm that isocyanuric acid and chlorine addition to thediene rubber molecules at the core surface has indeed occurred.

Following surface treatment, when the material at the surface portion ofthe solid core is examined by differential scanning calorimetry (DSC),no exothermic or endothermic peaks are observed from room temperature to300° C. This means that the functional groups which have been introducedmaintain a stable state within this temperature range. In other words,during molding of the cover material, the functional groups which havebeen introduced do not undergo degradation or the like due to heat, andcontinue to be effective. Also, because melting in the manner of a hotmelt resin does not arise, deleterious effects on durability and qualityof appearance, such as resin bleed out to the parting line, do notoccur. In addition, the very fact that the material in the surfaceportion of the solid core following the surface treatment describedabove is stable may be regarded as evidence that the isocyanuric acidhaving a melting point above 300° C. has been added with its molecularstructure still intact.

Next, the material making up the cover which directly encases the coreis described.

No particular limitation is imposed on the cover resin material in thisinvention, provided the material has a breaking strength of from 20 MPato 80 MPa and an elongation is of from 150% to 600%. However, preferreduse may be made of a thermoplastic resin such as an ionomer resin orpolyurethane. The use of a resin material composed primarily ofpolyurethane is especially preferred. For example, use may be made of athermoplastic polyurethane elastomer or a thermoset polyurethane resin,with the use of a thermoplastic polyurethane elastomer being especiallypreferred.

The breaking strength of the cover resin material is at least 20 MPa,preferably at least 25 MPa, more preferably at least 30 MPa, and mostpreferably at least 35 MPa, but not more than 80 MPa, preferably notmore than 75 MPa, more preferably not more than 70 MPa, and mostpreferably not more than 65 MPa. The elongation of the cover resinmaterial is at least 150%, preferably at least 200%, more preferably atleast 250%, and most preferably at least 300%, but not more than 600%,preferably not more than 550%, more preferably not more than 520%, andmost preferably not more than 490%. The breaking strength and elongation(tensile tests) are measured by methods in general accordance with JIS K7311-1995. By using such a cover resin material having a breakingstrength and an elongation in the above-indicated ranges, the durabilityto cracking, durability to surface loss and durability to abrasiondesired of a practice golf ball intended for long-term use can beimproved.

The thermoplastic polyurethane elastomer has a structure composed ofsoft segments formed from a polymeric polyol (polymeric glycol) and hardsegments formed from a chain extender and a diisocyanate. Here, thepolymeric polyol serving as a starting material may be any which hashitherto been used in the art relating to thermoplastic polyurethanematerials, and is not subject to any particular limitation. Exemplarypolymeric polyols include polyester polyols and polyether polyols.Polyether polyols are more preferable than polyester polyols becausethermoplastic polyurethane materials having a high rebound resilienceand excellent low-temperature properties can be synthesized.Illustrative examples of polyether polyols include polytetramethyleneglycol and polypropylene glycol. Polytetramethylene glycol is especiallypreferred from the standpoint of the rebound resilience and thelow-temperature properties. The polymeric polyol has an averagemolecular weight of preferably from 1,000 to 5,000. To synthesize athermoplastic polyurethane material having a high rebound resilience, anaverage molecular weight of from 2,000 to 4,000 is especially preferred.

The chain extender employed is preferably one which has hitherto beenused in the art relating to thermoplastic polyurethane materials.Illustrative examples include, but are not limited to, 1,4-butyleneglycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and2,2-dimethyl-1,3-propanediol. These chain extenders have an averagemolecular weight of preferably from 20 to 15,000.

The diisocyanate employed is preferably one which has hitherto been usedin the art relating to thermoplastic polyurethane materials.Illustrative examples include, but are not limited to, aromaticdiisocyanates such as 4,4′-diphenylmethane diisocyanate, 2,4-toluenediisocyanate and 2,6-toluene diisocyanate, and aliphatic diisocyanatessuch as hexamethylene diisocyanate. Depending on the type of isocyanate,control of the crosslinking reaction during injection molding may bedifficult. In this invention, the use of 4,4′-diphenylmethanediisocyanate, which is an aromatic diisocyanate, is most preferred.

A commercial product may be advantageously used as the thermoplasticpolyurethane material composed of the above materials. Illustrativeexamples include those available under the trade names Pandex T8180,Pandex T8195, Pandex T8290, Pandex T8295 and Pandex T8260 (all availablefrom DIC Bayer Polymer, Ltd.), and those available under the trade namesResamine 2593 and Resamine 2597 (available from Dainichiseika Color &Chemicals Mfg. Co., Ltd.).

The cover has a thickness which is preferably at least 0.3 mm, morepreferably at least 0.5 mm, and even more preferably at least 0.7 mm,but is preferably not more than 2.5 mm, more preferably not more than2.1 mm, even more preferably not more than 1.9 mm, still more preferablynot more than 1.8 mm, and most preferably not more than 1.7 mm. If thecover thickness is larger than the above range, the ball rebound maydecrease and the flight performance may worsen. On the other hand, ifthe cover thickness is smaller than the above range, the durability tocracking may decrease. In particular, when the ball is hit thin, or“topped,” the cover may tear.

The cover has a specific gravity which is preferably at least 1.13, morepreferably at least 1.14, and even more preferably at least 1.15, but ispreferably not more than 1.30, more preferably not more than 1.20, andeven more preferably not more than 1.17.

The cover material has a Shore D hardness which is preferably at least30, more preferably at least 35, and even more preferably at least 38,but is preferably not more than 57, more preferably not more than 55,even more preferably not more than 53, still more preferably not morethan 51, and most preferably not more than 48. If the Shore D hardnessof the cover is higher than the above range, the appearance performancein long-term use (durability of markings) may decline, in addition towhich the flight performance may markedly decrease. On the other hand,if the Shore D hardness of the cover is lower than the above range, thedurability to cracking may markedly decrease and, particularly when theball is topped, the cover may tear. In addition, the spin rate maybecome very high, possibly shortening the distance traveled by the ball.

The practice golf ball of the invention typically has numerous dimplesformed on the surface thereof, each dimple having a spatial volume belowa flat plane circumscribed by an edge of the dimple. Although notsubject to any particular limitation, the sum of the dimple spatialvolumes, expressed as a percentage (VR) of the volume of a hypotheticalsphere representing the ball were the ball to have no dimples on thesurface thereof, is preferably in a range of from 0.8% to 1.7%, thelower limit being more preferably 0.83%, even more preferably 0.85%, andmost preferably 0.86%, and the upper limit being more preferably 1.5%,even more preferably 1.3%, and most preferably 1.2%.

Also, although not subject to any particular limitation, the dimplesformed on the practice golf ball of the invention preferably satisfyconditions (1) and (2) below. Although it is preferable for both of thefollowing conditions (1) and (2) to be satisfied at the same time, it isacceptable for either one of these conditions alone to be satisfied.

First, referring to FIG. 3, as condition (1), it is preferable for thedimples to have a peripheral edge provided with a roundness representedby a radius of curvature R in a range of from 0.5 mm to 2.5 mm. Thelower limit of the radius of curvature R is more preferably 0.6 mm, andeven more preferably 0.7 mm, and the upper limit is more preferably 1.8mm, and even more preferably 1.5 mm.

Next, as condition (2), it is preferable for the ratio ER of acollective number of dimples RA having a radius of curvature R todiameter D ratio (R/D) of at least 20%, divided by a total number ofdimples N on the surface of the ball, to be in a range of from 15% to95%. Here, the ratio R/D is expressed as a percentage (R/D×100%), alarger value indicating a dimple in which the rounded part of the dimpleaccounts for a larger proportion of the dimple size and which has asmoother cross-sectional shape. The ratio ER indicates the number ofsuch smooth dimples as a proportion of the total number of dimples; bysetting ER in a range of from 15% to 95%, damage to the paint film atdimple edges can be effectively suppressed. The upper limit in the ratioR/D, although not subject to any particular limitation, is preferablynot more than 60%, and more preferably not more than 40%. The lowerlimit in the ratio ER is more preferably 20%, and even more preferably25%, and the upper limit is more preferably 90%, even more preferably85%, and most preferably 70%.

Also, although not subject to any particular limitation, it ispreferable for condition (3) to be satisfied. That is, as condition (3),it is preferable for the ball to have thereon a plurality of dimpletypes of differing diameter, and for the ratio DER of a combined numberof dimples DE obtained by adding together dimples having an own diameterand an own radius of curvature larger than or equal to a radius ofcurvature of dimples of larger diameter than the own diameter plusdimples of a type having a largest diameter, divided by the total numberof dimples N on the surface of the ball, to be at least 80%.

Generally, at a fixed dimple depth (see FIG. 3), the radius of curvatureR representing the roundness provided to the peripheral edges of thedimples is smaller at smaller dimple diameters D. However, abovecondition (3), by such means as adjusting the depth, sets the radius ofcurvature R representing the roundness of the peripheral edge to be aslarge as possible even in dimples having a small diameter D, thusforming dimples having a smooth cross-sectional shape and, by settingthe above ratio DER to at least 80%, increases the proportion of suchsmooth dimples, more effectively suppressing damage to the paint film.The ratio DER is more preferably at least 85%, even more preferably atleast 90%, and most preferably at least 93%. The upper limit in theratio DER is 100%.

In addition, the dimples formed on the practice golf ball of theinvention, although not subject to any particular limitation, preferablysatisfy conditions (4) to (6) below. Although it is preferable for allof the following conditions (4) to (6) to be satisfied at the same time,it is acceptable for any one of these conditions alone to be satisfied.

First, as condition (4), it is preferable for the number of dimple typesof differing diameter D on the ball to be 3 or more, and more preferablefor dimples of at least five types to be formed. In this case, thediameters D of the dimples, although not subject to any particularlimitation, are preferably set in a range of from 1.5 mm to 7 mm, thelower limit being more preferably 1.8 mm and the upper limit being morepreferably 6.5 mm. The depths of the dimples, although likewise notsubject to any particular limitation, are preferably set in a range offrom 0.05 mm to 0.35 mm, the lower limit being more preferably 0.1 mmand the upper limit being more preferably 0.3 mm, and even morepreferably 0.25 mm.

As condition (5), the total number of dimples N on the surface of theball is preferably not more than 380, and more preferably not more than350. The total number of dimples N is even more preferably in a range offrom 220 to 340.

As condition (6), it is preferable for the dimples to be formed in sucha way that the surface coverage SR of the dimples, which is the sum ofindividual dimple surface areas, each defined by a flat planecircumscribed by an edge of the dimple (dash-dot line in FIG. 3),expressed as a percentage of the surface area of a hypothetical sphererepresenting the ball were the ball to have no dimples on the surfacethereof (broken line in FIG. 3), is in a range of from 60% to 74%. At asurface coverage SR greater than 74%, the intervals between neighboringdimples become too narrow, which may make it difficult to provide thedimple edges with a roundness having the radius of curvature R specifiedin above condition (1). On the other hand, at a surface coverage SRbelow 60%, the aerodynamic performance decreases, as a result of whichthe distance traveled by the ball may decrease. The surface coverage SRhas a lower limit of more preferably 65%, and even more preferably 68%,and an upper limit of more preferably 73%.

In one-piece golf balls, because rubber has a somewhat yellow color, awhite enamel paint is generally applied as a first coat, following whicha clear paint is applied. As a result, a white paint cracks and dirt iseasy to adhere to the ball surface for a log period of time. In thepresent invention, the durability to cracking of the entire ball isenhanced by using polyurethane as the cover material, and a goodappearance in long-term use can be maintained without causing theproblems on cracking of a paint because the material itself for forminga cover is whiten to finish the ball surface with only a transparentclear paint as a paint layer. Thus, in the inventive ball, it ispreferable to apply a clear paint to the surface of the ball. Theresulting clear coat has a thickness at dimple lands (Y) which ispreferably at least 10 μm, more preferably at least 12 μm, and mostpreferably at least 13 μm, but is preferably not more than 30 μm, morepreferably not more than 25 μm, and most preferably not more than 20 μm;and a thickness at dimple edges (Z) which is preferably at least 8 μm,more preferably at least 10 μm, and most preferably at least 11 μm, butis preferably not more than 28 μm, more preferably not more than 23 μm,and most preferably not more than 18 μm. Also, the ratio Z/Y of edgeareas (Z) to land areas (Y), expressed as a percentage, is preferably atleast 60%, more preferably at least 70%, and most preferably at least80%, but is preferably not more than 100%, and more preferably not morethan 95%. Outside of the above range, the durability of markings atdimple edges decreases markedly in long-term use.

The ball diameter is preferably at least 42 mm, more preferably at least42.3 mm, and even more preferably at least 42.67 mm, but is preferablynot more than 44 mm, more preferably not more than 43.8 mm, even morepreferably not more than 43.5 mm, and most preferably not more than 43mm.

The ball weight is preferably at least 44.5 g, more preferably at least44.7 g, even more preferably at least 45.1 g, and most preferably atleast 45.2 g, but is preferably not more than 47.0 g, more preferablynot more than 46.5 g, and even more preferably not more than 46.0 g.

The ball deflection (referred to here and below as “BH”), whencompressed under a final load of 1,275 N (130 kgf) from an initial loadof 98 N (10 kgf), is preferably at least 2.0 mm, more preferably atleast 2.3 mm, and even more preferably at least 2.4 mm, but ispreferably not more than 7.0 mm. Moreover, when the core and ball areeach compressed under a final load of 1,275 N (130 kgf) from an initialload of 98 N (10 kgf), the ratio CH/BH of the respective deflections CHand BH is preferably at least 0.95, more preferably at least 0.96, andeven more preferably at least 0.97, but is preferably not more than 1.1,more preferably not more than 1.08, and even more preferably not morethan 1.07. If the ratio CH/BH is too large, the hardness (deflection) ofthe ball obtained as the finished ball will be very hard relative to thecore hardness (deflection).

That is, because the cover becomes harder, the feel on impact maydecrease and the quality of the appearance may decline with long-termuse. On the other hand, if the ratio CH/BH is too small, the cover willbe very soft, which may significantly lower the durability to crackingand lead to cracking of the cover, particularly when the ball is topped.In addition, the spin rate may undergo a large increase, which mayresult in a shorter distance of travel by the ball.

Also, in this invention, the ball rebound (BV) has an upper limit ofpreferably not more than 76.0 m/s, more preferably not more than 75.7m/s, even more preferably not more than 75.4 m/s, and most preferablynot more than 75.0 m/s. The lower limit is preferably at least 65 m/s,more preferably at least 68 m/s, even more preferably at least 71 m/s,and most preferably at least 73 m/s. At a ball rebound (BV) in excess of76.0 m/s, the ball may fly so well that is passes over the netting at adriving range, thus making the ball unsuitable for use as a practicegolf ball having a distance controlled for the driving range.

EXAMPLES

The following Examples and Comparative Examples are provided toillustrate the invention, and are not intended to limit the scopethereof.

Examples 1 to 4, Comparative Examples 1 to 4

Rubber materials formulated as shown in Table 1 below were furnished forthe fabrication of practice golf balls in the Examples and ComparativeExamples. These rubber compositions were suitably mixed using a kneaderor roll mill, then vulcanized under the temperature and time conditionsin Table 1 to produce solid cores in the respective Examples. Ingredientamounts in the table below are shown in parts by weight.

TABLE 1 Type of Core (1) (2) (3) (4) (5) (6) (7) Core BR01 100 60 60 95100 100 95 formulation IR2200 5 5 BR51 40 40 Perhexa C-40 0.6 0.6 (40%dilution) Actual amount 0.24 0.24 of addition Percumyl D 1.07 1.07 0.80.8 0.6 0.6 1.07 Zinc oxide 23 23 23 23 6 6 23 Antioxidant 0.2 0.2 0.20.2 0.2 0.2 0.2 Methacrylic acid 22.5 22.5 18 22.5 22.5 Zincmethacrylate 33 Zinc acrylate 33 Titanium oxide 4 VulcanizationTemperature (° C.) 170 170 170 170 160 160 170 conditions Time (minutes)20 20 20 20 13 13 30

Details on the materials used in the core formulations in the abovetable are provided below.

-   (1) BR01: A butadiene rubber synthesized with a nickel catalyst,    available from JSR Corporation; Mooney viscosity ML, 46-   (2) IR2200: An isoprene rubber, available from JSR Corporation;    Mooney viscosity ML, 82-   (3) BR51: A butadiene rubber synthesized with a neodymium catalyst,    available from JSR Corporation; Mooney viscosity ML, 36-   (4) Perhexa C-40: An organic peroxide, available from NOF    Corporation-   (5) Percumyl D: An organic peroxide, available from NOF Corporation-   (6) Zinc oxide: Available from Sakai Chemical Co., Ltd.-   (7) Antioxidant: “Nocrac NS-6,” available from Ouchi Shinko Chemical    Industry Co., Ltd.-   (8) Methacrylic acid: Available from Kuraray Co., Ltd.-   (9) Zinc methacrylate: Available from Asada Chemical Industry Co.,    Ltd.-   (10) Zinc acrylate: Available from Nihon Jyoryu Kogyo Co., Ltd.-   (11) Titanium oxide: Available from Ishihara Sangyo Kaisha, Ltd.

In each example, after the rubber composition formulated from theingredients shown in Table 1 was molded and vulcanized to form a core,the surface of the core was abraded to a desired diameter. Next, surfacetreatment of the core was carried out by immersing the core for 30seconds in an acetone solution of trichloroisocyanuric acid(concentration, 3 wt %), then washing the surface of the core withwater. The core was then set in a mold for injection molding the cover,and the cover composition shown in Table 2 below was injection moldedover the solid core. Ingredient amounts in the table below are shown inparts by weight.

TABLE 2 A Resin Pandex T8195 100 Additives Titanium dioxide 3.5Polyethylene wax 1.5 Shore D hardness 45

Details on the materials used in the cover composition in the abovetable are provided below.

-   “Pandex”: A thermoplastic polyurethane elastomer available under    this trade name from Dainippon Ink & Chemicals, Inc.-   Titanium dioxide: Available under the trade name “Tipaque R550” from    Ishihara Sangyo Kaisha, Ltd.-   Polyethylene wax: Available under the trade name “Sanwax 161P” from    Sanyo Chemical Industries, Ltd.

In order to form a predetermined dimple pattern on the surface of thecover, a plurality of protrusions corresponding to the dimple patternwere formed in the mold cavity, by means of which dimples were impressedonto the surface of the cover at the same time that the cover wasinjection molded. Details on the dimples are given below in Table 3. Themarkings shown in FIG. 5 were printed on the ball surface.

In addition, the ball was clear-coated with a paint composed of 100parts by weight of polyester resin (acid value, 6; hydroxyl value, 168)(solids)/butyl acetate/PMA (propylene glycol monomethyl ether acetate)in a weight ratio of 70/15/15 as the base; 150 parts by weight of anon-yellowing polyisocyanate, specifically a hexamethylene diisocyanateadduct (available from Takeda Pharmaceutical Co., Ltd. as TakenateD-160N; NCO content, 8.5 wt %; solids content, 50 wt %) as the curingagent; and 150 parts by weight of butyl acetate. In Comparative Example4, a coating of white enamel paint was applied as a base coat for clearcoating.

TABLE 3 Dimple Diameter D Depth R R/D N RA ER DE DER SR VR No. Number(mm) (mm) (mm) ratio (number) (number) (%) (number) (%) (%) (%)Configuration Dimple I 1 24 4.4 0.182 0.75 17 338 102 30 330 98 72 0.86FIG. 4 2 204 4.2 0.175 0.8 19 3 66 3.6 0.165 0.8 22 4 12 2.7 0.135 0.933 5 24 2.5 0.105 0.9 36 6 8 3.4 0.145 0.6 18 Dimple II 1 24 4.4 0.2070.7 16 338 102 30 330 98 72 0.99 FIG. 4 2 204 4.2 0.200 0.7 17 3 66 3.60.190 0.8 22 4 12 2.7 0.160 0.85 31 5 24 2.5 0.130 0.85 34 6 8 3.4 0.1450.55 16 Dimple III 1 24 4.4 0.216 0.5 11 338 36 11 306 91 72 0.99 FIG. 42 204 4.2 0.209 0.5 12 3 66 3.6 0.194 0.6 17 4 12 2.7 0.151 0.6 22 5 242.5 0.116 0.5 20 6 8 3.4 0.160 0.5 15

The abbreviations and symbols relating to dimples which appear in Table3 are explained below.

-   R: Radius of curvature representing roundness provided at peripheral    edge of a dimple-   R/D ratio: Ratio of radius of curvature R to diameter D-   N: Total number of dimples on surface of ball-   RA: Collective number of dimples having an R/D ratio of at least 20%-   ER: Ratio of RA to total number of dimples N-   DE: Sum of number of dimples having an own diameter and an own    radius of curvature larger than or equal to a radius of curvature of    dimples of larger diameter than the own diameter, plus number of    dimples of a type having a largest diameter-   DER: Ratio of DE to total number of dimples N-   SR: Sum of individual dimple surface areas, each defined by a flat    plane circumscribed by an edge of the dimple, expressed as a    percentage of the surface area of a hypothetical sphere representing    the ball were the ball to have no dimples on the surface thereof.-   VR: Sum of individual dimple spatial volumes, each formed below a    flat plane circumscribed by an edge of the dimple, expressed as a    percentage of the volume of a hypothetical sphere representing the    ball were the ball to have no dimples on the surface thereof.

The physical properties of the cores and covers in the respectiveexamples of the invention and the comparative examples, and the physicalproperties, distance, durability and feel of the practice balls obtainedin each example were measured or evaluated as described below. Theresults are presented in Table 4.

Deflection of Core and Finished Ball (mm)

The deflection (mm) of the core or finished ball as the test sphere whencompressed at a rate of 10 mm/min under a final load of 1,275 N (130kgf) from an initial load state of 98 N (10 kgf) was measured. The testwas performed using a model 4204 test system from Instron Corporation.

Cross-Sectional Hardness of Core

The core was cut with a fine cutter and the JIS-C hardnesses at abovepositions B to F were measured in accordance with JIS K6301-1975 afterholding the core isothermally at 23±1° C. (at two places in each of N=5samples).

Surface Hardness of Core

JIS-C hardness measurements were carried out on the core surface inaccordance with JIS K6301-1975 after holding the core isothermally at23±1° C. (at two places in each of N=5 samples).

Rebound (Initial Velocity)

The initial velocity was measured using an initial velocity measuringapparatus of the same type as the USGA drum rotation-type initialvelocity instrument approved by the R&A. The balls were heldisothermally at a temperature of 23+1° C. for at least 3 hours, thentested in a room temperature (23±2° C.) chamber. Ten balls were each hittwice, and the time taken for the balls to traverse a distance of 6.28ft (1.91 m) was measured and used to compute the initial velocity.

Cover Material Hardness

A cover sheet was formed and, after holding the samples isothermally at23±1° C., the Shore D hardness was measured in accordance with ASTMD-2240.

Breaking Strength and Elongation (Tensile Tests)

The resin materials and rubber materials were formed into 2 mm thicksheets, and held in a 23+1° C. atmosphere for two weeks. These sampleswere shaped into dumbbell-shaped test specimens in accordance with JIS K7311-1995, and the specimens were subjected to measurement in a 23±2° C.atmosphere at a test rate of 5 mm/s, also in accordance with JIS K7311-1995. The average breaking strength and elongation of each materialwas calculated from the measured values for five specimens.

Measurement of Coating Thickness

-   Lands (Y): The thickness of the clear coat at land areas at    intermediate positions between dimples was measured.-   Edges (Z): The thickness of the clear coat at dimple edge areas was    measured.

The above measurements were carried out at three places on each of twoballs in the respective examples, and the average of these measurementswas determined.

Distance

A TourStage X-Drive 701 (loft angle, 9°), manufactured by BridgestoneSports Co., Ltd., was mounted as the driver (W#1) on a golf swing robotand struck at a head speed (HS) of 45 m/s. Both the spin rate of theball immediately after impact and the total distance traveled by theball were measured.

In addition, after the abrasion test described below had been carriedout, the total distance of the ball was again measured.

Durability to Cracking

The ball was hit five times at the same place with the leading edge of anumber nine iron (X-BLADE GR, manufactured by Bridgestone Sports Co.,Ltd.) at a head speed (HS) of 38 m/s, following which the ball wasrepeatedly struck against a wall at an incident velocity of 43 m/s andthe average number of shots (N=3 balls) until cracking occurred wasdetermined (evaluation of durability when ball is topped).

Abrasion Test

Ten golf balls and 3 liters of bunker sand were placed in a magneticball mill having an 8 liter capacity and mixing was carried out for 144hours, following which the balls were visually examined for any loss ofthe markings and to assess the degree of surface scratching, the degreeof loss of luster and the degree of sand adhesion. The ball appearancewas rated as “good,” “fair” or “NG.”

TABLE 4 Example Comparative Example 1 2 3 4 1 2 3 4 Core Type (1) (2)(3) (4) (5) (6) (6) (7) Diameter, mm 39.9 39.9 39.9 39.9 39.9 39.9 39.942.7 Specific gravity 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 Deflectionunder 10-130 kg 2.6 2.5 3.8 3.1 2.75 2.75 2.75 compression (CH), mmJIS-C hardness 81 82 69 72 80 80 80 80 at core surface (A) JIS-Chardness 2 mm 76 77 63 64 75 75 75 75 inside core surface (B) JIS-Chardness 5 mm 79 79 67 69 77 77 77 77 inside core surface (C) JIS-Chardness 10 mm 74 74 66 69 71 71 71 71 inside core surface (D) JIS-Chardness 15 mm 69 69 64 67 67 67 67 67 inside core surface (E) JIS-Chardness 66 65 61 64 63 63 63 63 at core center (F) JIS-C hardnessdifference 2 3 2 3 3 3 3 3 between core surface and 5 mm inside coresurface (A − C) JIS-C hardness difference 15 17 8 8 17 17 17 17 betweencore surface and center (A − F) Cover Type A A A A A A A Shore Dhardness 45 45 45 45 45 45 45 Breaking strength, MPa 40 40 40 40 40 4040 Elongation, % 360 360 360 360 360 360 360 Specific gravity 1.15 1.151.15 1.15 1.15 1.15 1.15 Thickness, mm 1.4 1.4 1.4 1.4 1.4 1.4 1.4Finished Deflection under 10-130 kg 2.65 2.55 3.8 3 2.75 2.75 2.75 2.75ball loading (BH), mm Rebound (BV), m/s 74.2 74.3 74.1 73.5 76.2 76.676.6 74.6 Diameter, mm 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Coredeflection/Ball deflection (CH/BH) 0.98 0.98 1.00 1.03 1.00 1.00 1.00Dimples Type I I II II I I III I Clear Land areas (Y), μm 16 16 15 15 1616 17 16 coating Edge areas (Z), μm 14 14 13 13 14 14 8 14 thicknessCoating thickness ratio 88 88 88 88 88 88 47 88 (Z/Y × 100), % DistanceHS 45, Spin rate, rpm 3370 3380 3090 3220 3360 3340 3340 3650 driverTotal distance, m 223 223 215 213 233 235 235 221 HS 45, Total distance,m 220 220 212 211 230 232 228 215 driver (after abrasion test) DistanceTotal distance, m −3 −3 −3 −2 −3 −3 −7 −6 difference DurabilityDurability At incident 1103 1101 1050 1095 615 579 579 621 to crackingvelocity of 43 m/s Abrasion After 144 hours of good good good good goodgood NG NG test abrasion with sand

The rubber material in Comparative Example 4 had a breaking strength of15 MPa and an elongation of 88%.

The practice golf ball in Comparative Example 1 included zincmethacrylate as the co-crosslinking agent in the core-forming rubberformulation. As a result, the durability to cracking was poor.

The practice golf ball in Comparative Example 2 included zinc acrylateas the co-crosslinking agent in the core-forming rubber formulation. Asa result, the durability to cracking was very poor.

In the practice golf ball in Comparative Example 3, zinc acrylate wasincluded in the rubber composition and the cover was composed primarilyof an ionomer, as a result of which the durability to cracking was verypoor. In addition, because the cover was composed primarily of anionomer, the cover was too hard. As a result, the durability of markingswas very poor and the flight performance following the abrasion test(following evaluation of the durability of markings) decreased markedly.

The practice golf ball of Comparative Example 4 was a one-piece golfball composed of a rubber layer. Because rubber was used, the breakingstrength was poor and the elongation was low. As a result, thedurability to cracking, particularly the durability to cutting, waspoor, in addition to which surface loss readily arose. Also, in theabrasion test, since a clear paint was formed on the coating of whiteenamel paint, cracking occurs to the white paint and dirty was easy toadhere to its surface so that the appearance was poor when seen theappearance after the abrasion test. Moreover, the decrease in flightperformance following the abrasion test (following evaluation of thedurability of markings) was somewhat large, and the spin rate increased.

Japanese Patent Application No. 2011-099853 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A practice golf ball comprising: a core made of a rubber compositioncomprising a base rubber and, as compounding ingredients: aco-crosslinking agent, a crosslinking initiator and a metal oxide; and acover which encases the core and is made of a resin material, whereinthe co-crosslinking agent is methacrylic acid, and the resin materialhas a breaking strength of from 20 MPa to 80 MPa and an elongation offrom 150% to 600%.
 2. The practice golf ball of claim 1, wherein theball has an initial velocity of not more than 76 m/s.
 3. The practicegolf ball of claim 1, wherein the resin material making up the cover iscomposed primarily of polyurethane, the cover material has a Shore Dhardness of from 30 to 57, and the cover has a thickness of from 0.3 mmto 2.5 mm.
 4. The practice golf ball of claim 1, wherein the ball hasformed on a surface thereof a plurality of dimples, each dimple having aspatial volume below a flat plane circumscribed by an edge of thedimple, and the sum of the dimple spatial volumes, expressed as apercentage (VR) of the volume of a hypothetical sphere representing theball were the ball to have no dimples on the surface thereof, being from0.8% to 1.7%.
 5. The practice golf ball of claim 1, wherein the ball hasformed on a surface thereof a plurality of dimples which satisfyconditions (1) and (2) below: (1) the dimples have a peripheral edgeprovided with a roundness represented by a radius of curvature R of from0.5 mm to 2.5 mm; and (2) the ratio ER of a collective number of dimplesRA having a radius of curvature R to diameter D ratio (R/D) of at least20%, divided by a total number of dimples N on the surface of the ball,is from 15% to 95%.
 6. The practice golf ball of claim 5 which furthersatisfies condition (3) below: (3) the ball has thereon a plurality ofdimple types of differing diameter, and the ratio DER of a combinednumber of dimples DE obtained by adding together dimples having an owndiameter and an own radius of curvature larger than or equal to a radiusof curvature of dimples of larger diameter than said own diameter plusdimples of a type having a largest diameter, divided by the total numberof dimples N on the surface of the ball, is at least 80%.
 7. Thepractice golf ball of claim 6 which further satisfies conditions (4) to(6) below: (4) the number of dimple types of differing diameter is 3 ormore; (5) the total number of dimples N is not more than 380; and (6)the surface coverage SR of the dimples, which is the sum of individualdimple surface areas, each defined by a flat plane circumscribed by anedge of the dimple, expressed as a percentage of the surface area of ahypothetical sphere representing the ball were the ball to have nodimples on the surface thereof, is from 60% to 74%.
 8. The practice golfball of claim 1, wherein the core has a hardness profile in which,letting A be the JIS-C hardness at a surface of the core, B be the JIS-Chardness at a position 2 mm inside the core surface, C be the JIS-Chardness at a position 5 mm inside the core surface, D be the JIS-Chardness at a position 10 mm inside the core surface, E be the JIS-Chardness at a position 15 mm inside the core surface, and F be the JIS-Chardness at a center of the core: A is from 60 to 88, B is from 54 to83, C is from 56 to 85, D is from 54 to 80, E is from 51 to 75, and F isfrom 48 to 72; the relative hardness conditions A>B<C≧D>E>F aresatisfied; the value A−F is not more than 19; the core is formed in sucha way that A has the highest value among A to F; the value A−C is from 0to 8; the core has a specific gravity of from 1.05 to 1.2; and, when thecore and the ball are each compressed under a final load of 1,275 N (130kgf) from an initial load of 98 N (10 kgf), letting deflection by thecore be CH and deflection by the ball be BH, the core deflection CH isfrom 2.0 mm to 7.0 mm, the ball deflection BH is from 2.0 mm to 7.0 mm,and the ratio CH/BH is from 0.95 to 1.1.
 9. The practice golf ball ofclaim 1 wherein the metal oxide is zinc oxide, and the compoundingingredients are included in respective amounts of from 10 to 40 parts byweight of methacrylic acid, from 0.3 to 5.0 parts by weight ofcrosslinking initiator, and from 15 to 30 parts by weight of zinc oxide,per 100 parts by weight of the base rubber.